Habitat and Resource Conditions

The likelihood that an insect will find a suitable patch depends strongly on patch size and proximity to insect population sources. The probability of survival declines with distance, as a result of depletion of metabolic resources and protracted exposure to various mortality factors (Pope et al. 1980). Hence, more insects reach closer resources or sites. Sartwell and Stevens (1975) and Schowalter et al. (1981b) reported that, under nonoutbreak conditions, probability of bark beetle, Dendroctonus spp., colonization of living pine trees declined with distance from currently attacked trees. Trees more than 6 m from currently colonized trees had negligible probability of colonization by sufficient numbers to successfully kill the tree. Under outbreak conditions, the effect of distance disappeared (Schowalter et al. 1981b). Similarly, He and Alfaro (1997) reported that, under nonoutbreak conditions, colonization of white spruce by the white pine weevil, Pissodes strobi, depended on host condition and distance from trees col onized the previous year, but during outbreaks most trees were sufficiently near occupied trees to be colonized.

Larger or more conspicuous habitats or resources are more likely to be perceived by dispersing insects or to be intercepted by a given direction of flight (see Chapter 7). For example, Courtney (1985,1986) reported that the pierid butterfly, Anthocharis cardamines, preferentially oviposited on the most conspicuous (in terms of flower size) host species that were less suitable for larval development than were less conspicuous hosts. This behavior by the adults represented a tradeoff between the prohibitive search time required to find the most suitable hosts and the reduced larval survival on the most conspicuous hosts. Larger habitat patches also intersect a longer arc centered on a given starting point. Insects dispersing in any direction have a higher probability of contacting larger patches than they do smaller patches.

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